Recently, due to growing worldwide awareness of the environment, it has been strongly demanded in the automotive field to reduce the carbon dioxide emission or improve fuel consumption. For solving these tasks, weight reduction of a vehicle body may be effective, and application of a high-strength steel sheet may be being promoted to achieve the weight reduction. At present, a hot-rolled steel sheet with a tensile strength of a 440 MPa level may be often used for automotive underbody components. Despite the demand for application of a high-strength steel sheet so as to cope with the weight reduction of a vehicle body, a hot-rolled steel sheet having a tensile strength of 500 MPa or more may currently settle for its application to a part of the components. Main causes thereof may include deterioration of press formability associated with an increase in strength.
Many underbody members of an automobile may have a complicated shape to ensure high rigidity. In press forming, various kinds of workings such as burring, stretch flanging and stretching may be applied and therefore, workability responding to these works may be required of the hot-rolled steel sheet as a blank. In general, the burring workability and the stretch flanging workability may be considered to have a correlation with a hole expanding ratio measured in a hole expanding test, and development of a high-strength steel sheet improved in the hole expandability has been heretofore advanced.
As for the measure to enhance the hole expandability, it is said that elimination of a second phase or an inclusion in the structure of a hot-rolled steel sheet may be effective. The plastic deformability of such a second phase or an inclusion may significantly differ from that of the main phase and therefore, when a hot-rolled steel sheet is worked, stress concentration may occur at the interface between the main phase and the second phase or inclusion. In turn, a fine crack working out to a starting point for fracture may be readily generated at the boundary between the main phase and the second phase or inclusion. Accordingly, it may greatly contribute to enhancement of hole expandability to limit the amount of a second phase or an inclusion and thereby reduce the starting point for crack generation as much as possible.
For these reasons, a hot-rolled steel sheet with excellent hole expandability may be ideally a single-phase structure steel, and in a dual-phase structure steel, the difference in the plastic deformability between respective phases constituting the dual-phase structure may be preferably small. That is, it is preferable that the hardness difference between respective phases is small. As the hot-rolled steel sheet excellent in hole expandability in line with such a way of thinking, a steel sheet having a structure mainly composed of bainite or bainitic ferrite has been proposed (for example, Patent Document 1).